The DSTWO is a bit expensive because of it's extra hardware which only a few flashcarts have (there are only 3 that have built in CPUs and the DSTWO is the only cart that seems to use it effectively.) Right now it's the flashcart with the highest compatibility because it's built in CPU which emulates a game cartridge; most games that come out with new AP are bypassed easily because of this. Not only do you get a flashcart with great NDS compatibility, but you get access to homebrew which isn't usable in any other cart (e.g. MAME, GBA, SNES, Video Player). The only problem you might run into is that there will be a notable decrease in battery life which you get 20-30 minutes scraped off your DS's run time.

If your plan on sticking with your budget, and don't mind waiting for an AP patch or a new update, I'd say either an Acekard or an R4i Gold since it's now getting updates from YWG (Wood Firmware). They both have great compatibility, and pretty much work with every game that out right now; just don't expect newly released games to work right off the spot.